This invention relates to a novel radiation-sensitive resin composition which can be employed as a material for forming a resist used in the manufacture of semiconductor devices, and which is sensitive to light, electron beams, X rays or ion beams.
Due to the increasing packing density of LSI devices, techniques are coming to be required which can process components at the sub-micron level. One such technique used in the manufacture of LSI is dry etching, a procedure which permits high precision and fine detail.
The meaning of dry etching in this context is a procedure whereby a substrate (a semiconductor substrate or any layer thereon) to be processed is covered by a resist, the resist is patterned by light or an electron beam, and using the pattern obtained as a mask, the exposed parts of the substrate not covered by the mask are etched by a reactive gas plasma. Resists used in dry etching therefore consist of materials which are able to resolve patterns of sub-micron order, and which have sufficient resistance to reactive gas plasmas.
As LSI devices become more highly integrated, the steps formed on the substrates are becoming higher, and the patterns formed on the substrate are required to have high aspect ratios. In order for the steps to be planarized, and in order for the resist layer to serve as a mask right up to the end of processing, the resist layer is tending to be made thicker. This however means that when the resist is exposed to light, any restriction in the depth of focus of the exposing optical system will have more adverse effect, while if the resist is exposed to an electron beam, electron scattering has more serious consequences. This leads to a decrease in resolution of the resist, and as a result, it is becoming increasingly difficult to process a substrate with a single resist layer.
A new resist process that is now being studied is the double layer resist method. This makes use of a thick polymer layer (referred to hereinafter as a lower layer normally consisting of a polyimide or thermally cured photoresist) for planarizing the steps in the substrate, and a thin photoresist layer or electron beam resist layer (referred to hereinafter as an upper layer) which is formed on the polymer layer and has resistance to O.sub.2 -RIE (reactive ion etching). More specifically, the upper layer makes it possible to obtain a high resolution pattern which is used as a mask to pattern the lower layer by O.sub.2 -RIE, and the lower layer pattern thus obtained is then used as a mask for dry etching when the substrate is processed.
The upper layer resist must have a greater resistance to O.sub.2 -RIE than the lower layer resist, and it therefore often consists of a silicon type photosensitive resin composition. An example of such a composition is a silicone resin used as a binder with which a photo crosslinking agent or a photopolymerization initiator is blended, and this may be used as a negative resist.
The following are conventional examples of such a photosensitive resin composition.
Japanese Patent Kokai Publication S61-20030 discloses a composition comprising a resin containing double bonds such as poly(acryloyloxymethylphenylethylsilsesquioxane), and a diazide. This composition could be used as a highly sensitive resist for patterning by UV (ultra-violet) light in a nitrogen atmosphere.
Japanese Patent Kokoku Publication No. S60-49647 discloses a photosensitive resin composition using a polysilane as a photopolymerization initiator. More specifically, it is disclosed that a composition consisting of a poly(organosiloxane) having double bonds and dodecamethylcyclohexasilane has good properties as a UV photocuring resin.
Japanese Patent Kokai Publication S55-127023 discloses a photosensitive resin composition Which uses an organic peroxide as a photopolymerization initiator. More specifically, it is disclosed that a composition using a polyorganosiloxane containing double bonds and a suitable organic peroxide (for example a peroxyester or the like), gives a uniform cured film when irradiated by UV light.
The following completely different types of materials have been proposed for the photosensitive resin composition used as the upper layer.
Japanese Patent Kokai Publication S61-144639 discloses a composition wherein polyphenylsilsesquioxane and cis(1,3,5,7-tetrahydroxy)-1,3,5,7-tetraphenylcyclotetrasiloxane are added in small amounts to a common positive photoresist such as OFPR-800 (Tokyo Ooka Kogyo Inc.). This composition can be used as a positive resist which can be developed in alkali.
Other materials apart from polysiloxanes are being studied in view of their possible application as binders. Japanese Patent Kokai Publication S61-198151, for example, discloses a photosensitive resin composition sensitive to visible light which uses a novolak resin containing trialkylsilyl groups together with a diazonaphthoquinone photosensitizer.
If the aforesaid conventional photosensitive compositions were used as upper layer resists in a two layer resist process, however, the following problems arose:
(a) In general, photocuring reactions which proceed via organic radical intermediates are blocked by oxygen. In the case of a photosensitive resin composition which is photocured by this mechanism, therefore (for example, the photosensitive resin composition disclosed in Japanese Patent Kokai Publication S61-20030 wherein a diazide is used as photopolymerization initiator), high sensitivity could not be achieved unless the exposure were performed in a nitrogen atmosphere. The same problem arises in the case of the photosensitive resin composition disclosed in Japanese Patent Kokai Publication S55-127023 wherein an organic peroxide is used as photopolymerization initiator. When an organic peroxide was used in the composition, the properties of the cured film improved, however a considerable time was required for curing.
(b) Regarding the photosensitive resin composition disclosed in Japanese Patent Kokoku Publication No. S60-49647 wherein a polysilane is used as photopolymerization initiator, it is the inventor's experience that although a diazide is not used, sensitivity still deteriorates if oxygen is present.
Due to the aforesaid problems (a) and (b), a high throughput cannot be obtained if the exposure is performed in a nitrogen atmosphere. Moreover, performing the exposure in a nitrogen atmosphere requires various equipment to supply the nitrogen in the vicinity of the wafer.
(c) Regarding the photosensitive resin composition disclosed in Japanese Patent Kokai Publication S61-144639 wherein a silicon compound is added to a common photoresist, or the photosensitive resin composition disclosed in Japanese Patent Kokai Publication S61-198151 wherein a substance other than a polysiloxane is used as a binder, the silicon content of the silicon compound added and the silicon content of the binder used are too low so that sufficient O.sub.2 -RIE resistance is not obtained.